Certain capacitors are required to be provided with a pressure interrupter, that is, when pressure builds in the case due to a failure of the capacitor, the case expands, thereby causing a break in the electrical connection between the capacitor element and the terminals mounted on the case.
One type of interrupter is the “bridge” type interrupter, shown in Flanagan—U.S. Pat. No. 4,106,068. The capacitor terminals may be mounted on a flexible cover, which seals the capacitor case. Rivets on the bottom of the terminals extend through a relatively rigid plate mounted within the case, underneath the cover. Electrical conductors, such as tabs, connect the electrodes of the capacitor elements to the rivets on the underside of the plate, for example, by welding. When pressure builds within the case due to an internal fault, the cover bulges outward, causing the terminals to move away from the rigid plate, thereby breaking the connections between the tabs and the rivets.
An important consideration when a capacitor is installed in an electrical circuit is that there be sufficient clearance to allow the cover to expand and break the connections to the terminals. For example, approximately ½ inch or more of clearance is required between the terminals and any obstruction, to allow the cover to expand outward under pressure. For some applications, however, such as mounting the capacitor on a busbar, it may be difficult to maintain the clearance necessary for the pressure interrupter to function.
Another type of interrupter is the “bellows” type interrupter, shown in Deschanels et al. —U.S. Pat. No. 4,283,750. The body of the case may be provided with an accordion style pleat or collapsed section. When the pressure builds, the case lengthens, and the bottom of the case expands away from the cover. A wire connecting the capacitor element to a terminal mounted on the cover is secured to the capacitor element and is “nicked” or otherwise weakened in one place. When the case expands under pressure, the wire is pulled taut and breaks, thereby disconnecting the terminal from the capacitor element.
Erhardt et al.—U.S. Pat. No. 4,577,257 disclose a capacitor having a housing with a flexible bottom. An electrical conductor is connected at one end to the bottom of the housing, and the conductor is nicked to provide a predetermined breaking point. An increase in the pressure within the housing causes the bottom to flex outward and break the conductor.
A shortcoming of the pressure interrupters of Deschanels et al. and Erhardt et al. is that only the electrical connection between one electrode of the capacitor element and one terminal on the cover of the case is disconnected. Best practice for many applications, however, is that the electrical connections from the capacitor element(s) to all of the terminals be disconnected in the case of an internal fault.
Whether a bridge type or bellows type pressure interrupter is provided with the capacitor, mounting brackets, heavy cables and other structural members may restrict expansion of the capacitor cover or housing at a particular installation. Providing a capacitor with dual pressure interrupters, especially a capacitor having pressure interrupters that depend on the expansion of different components of the case to engage, would greatly decrease the risk of a rupture and venting under internal fault conditions.